Troodon had some of the largest known brains of any dinosaur group, relative to their body mass (comparable to modern birds).[9] Troodon's cerebrum-to-brain-volume ratio was 31.5% to 63% of the way from a nonavian reptile proportion to a truly avian one.[10] Troodon had bony cristae supporting their tympanic membranes that were ossified at least in their top and bottom regions. The rest of the cristae were either cartilaginous or too delicate to be preserved. The metotic strut of Troodon was enlarged from side-to-side, similar to Dromaeosaurus and primitive birds like Archaeopteryx and Hesperornis.[10]